This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-110830, filed Apr. 19, 1999; and No. 11-322496, filed Nov. 12, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an optical information processing apparatus for carrying out image processing or image recognition using a reflection type spatial light modulator, for example, and more particularly to an optical information processing apparatus having a simplified and compact structure achieved by improving an optical system for use therein.
Further, the present invention relates to an optical information processing apparatus and more particularly, an optical information processing apparatus for optically carrying out image processing or image recognition using a reflection type spatial light modulator (hereinafter referred to as SLM) for filter display.
As well known, two-dimensional image Fourier transformation which requires a large amount of computation can be achieved rapidly by means of a single lens if optical means is used. Thus, since before, various researches on rapid optical information processing such as correlation, convolution, filtering and the like have been carried out.
FIG. 16 shows an example of general optical information processing apparatus optical system.
That is, as shown in FIG. 16, light emitted from a coherent light source 10 is condensed by a condensing lens 11, is focused on a spatial filter 12, and is filtered. Then the filtered light arrives to a collimator lens group 13 and is collimated. The collimated light arrives to a transmission type light modulator 14T in which the input image 141 is displayed and is modulated. The modulated light arrives to a Fourier transformation lens group 15, is Fourier transformed, and forms a Fourier transformation image 161 of the input image 141 on the rear focal plane (FB plane) of the Fourier transformation lens group 15. A filter 16 is provided on the rear focal plane of the Fourier transformation lens group 15 to process optical information of the Fourier transformation image 161 of the input image 141.
A Fourier inverse transformation lens group 17 is provided in a manner that a front focal plane (IEF) of the Fourier inverse transformation lens 17 locates on the filter 16. The light that passes filter 16 arrives to the Fourier inverse transformation lens 17 and is Fourier inverse transformed. Finally, the Fourier inverse transformed light arrives at an images pickup device 18 placed on the rear focal plane (IFB) of the Fourier inverse transformation lens 17. The processed result of image 181 is obtained by the image pickup device 18. Thus, the input image 141 is Fourier transformed, filtered, Fourier inverse transformed, and picked up.
FIG. 16 shows an opening as the input image 141 and indicates an example of processing for carrying out bypass filtering for its Fourier transformation image 161.
In this case, the Fourier transformation image 161 of the input image 141 is subjected to filtering by a filter 16 having a ring-like opening for extracting only high frequency components apart from the center thereof and consequently, the processing result image 181 whose boundary is emphasized is obtained on the image pickup device 18.
FIGS. 17, 18 show conventional cases where an input image is displayed on the reflection type spatial light modulator.
The optical information processing apparatus optical system shown in FIG. 17 includes a reflection type spatial light modulator 14R, a Fourier transformation lens group 15, a filter 16, a Fourier inverse transformation lens 17, an image pickup device 18. This optical information processing apparatus operates as the apparatus shown in FIG. 16.
Its structure is different from that in FIG. 16 in that a polarized beam splitter 19 is added to use the reflection type spatial light modulator 14R as a spatial light modulator.
This polarized beam splitter 19 allows P-polarized light (light polarized in parallel to paper surface, indicated by both end arrows in FIG. 17) to pass and reflects S-polarized light (light polarized to a plane perpendicular to paper surface, indicated by a symbol indicating a perpendicular direction in FIG. 17).
If collimated light just after it is emitted from the collimator lens group 13 is modulated to S-polarized light, it is reflected by a PBS plane 191 and then impinges upon the reflection type spatial light modulator 14R.
The reflection type spatial light modulator 14R expresses respective pixel values of an indicated input image 141 with an orientation direction of liquid crystal molecules in each pixel.
As a result, the entered collimator light is modulated so that its P-polarized light component is enlarged depending on the pixel value at the time of reflection.
That is, S-polarized light and P-polarized light are mixed in beam of light just after it is reflected by the reflection type spatial light modulator 14R.
When light in which S-polarized light and P-polarized light are mixed reaches the plane 191 of the polarized beam splitter 19, only the P-polarized light passes through. Consequently, the Fourier transformation image 161 of the input image 141 is generated on the FB plane at the rear focal plane of the Fourier transformation lens group 15.
A structure after the filter 16 is the same as shown in FIG. 16, so that finally, the processing result image 181 is obtained on the FF plane.
A disposition shown in FIG. 18 also produces the same function.
In this case, collimated light emitted from the collimator lens group 13 is modulated to P-polarized light and light for reading the input image 141 written in the reflection type spatial light modulator 14R is modulated to S-polarized light.
Because the conventional optical information processing apparatus optical system shown in FIGS. 17, 18 has a structure redundant in the axial direction, if considering its practical performance, it is important to achieve as compact a structure as possible, and further a high performance Fourier transformation lens group is required to increase the capacity of the image.
For example, Jpn. Pat. Appln. KOKAI Publication No. 5-88079 has disclosed a design example of a Fourier transformation lens group comprised of three groups whose power is distributed to positive, negative and positive as shown in FIG. 19.
A first group 151 is composed of cemented lens having positive power, a second group 152 is comprised of two meniscus lenses having negative power and a third group 153 is comprised of two meniscus lenses having positive power.
In this design example, by replacing the front main plane HF and rear main plane HB of the Fourier transformation lens group, a distance between the front focal plane FF and rear focal plane FB is 1.25f-less than 2f. In an optical information processing apparatus optical system using this, a length in the axial direction is short.
Although pixel size of the spatial light modulator has been decreased with a progress of technology, because the capacity of a handled image has increased, a display area size of a spatial light modulator having one million pixels, for example, is relatively large.
To illuminate this region with a uniform intensity, a lens having a high NA or a lens having a low NA but a long focal distance is required for the collimator lens group.
Because the focal distance is decreased if such a high NA collimator lens group is applied, apparently a distance in the axial direction required from the front focal plane up to a position in which a collimator light is obtained is thought to be decreased.
However, in this case, optical performance demanded for the collimator lens group becomes very high.
This results in increasing the number of the lens elements of the collimator lens group and the distance from the front focal plane CF of the collimator lens group up to a position in which the collimator light is obtained is not always decreased.
If a high NA collimator lens group is applied, an opening size of the spatial filter is desired to be small. In such a case, it is necessary to provide a portion containing the spatial filter with a high precision alignment mechanism.
This high precision alignment mechanism or high NA collimator lens group not only results in increase of production cost but also makes it difficult to construct a compact optical information processing apparatus optical system. In this case, a stabilized operation is not easy to ensure.
On the other hand, if a collimator lens group whose NA is not so high, but focal distance is long is used, the number of lenses is decreased and the alignment mechanism can be simplified, so that a low production cost can be achieved.
However, because the focal distance is long, the distance in the axial direction from the spatial filter up to a position in which the collimator light is obtained is naturally elongated.
Although an attention has to be paid to an entire balance including cost, alignment and performance when practical realization of the optical information processing apparatus optical system is considered, there are few cases in which the above matters have been considered sufficiently in the conventional optical information processing apparatus optical system employing a reflection type spatial light modulator for input image display.
As for the optical information processing apparatus for displaying the input image on the transmission type spatial light modulator, some inventions have been achieved in which the entire optical system is made compact by setting the distance between the front focal plane FF and rear focal plane FB of the Fourier transformation lens group to less than twice (2f) the focal distance, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-88079.
In the design example of Jpn. Pat. Appln. KOKAI Publication No. 5-88079, as shown in FIG. 19, the optical information processing apparatus optical system using the transmission type spatial light modulator has been designed considering its compactness.
However, this structure cannot be applied to the optical information processing apparatus optical system in which the polarized beam splitter 19 is disposed between the Fourier transformation lens group 15 and reflection type spatial light modulator 14R as shown in FIGS. 17, 18, because a distance between the front focal plane and first plane is short.
In design examples of the conventional Fourier transformation lens group, it is premised that the Fourier transformation lens group is used in an optical information processing apparatus optical system as shown in FIG. 16 which uses the transmission type spatial light modulator for input image display. There has been no concrete design example about the Fourier transformation lens group to be used in the optical information processing apparatus optical system using the reflection type spatial light modulator.
In the optical information processing apparatus optical system using the conventional reflection type spatial light modulator as shown in FIGS. 17, 18, there is no example in which its structure, performance, cost and alignment are considered sufficiently for both the collimator lens group 13 and Fourier transformation lens group 15.
Further, in the conventional example, the collimator lens group 13 and Fourier transformation lens group 15, which are independent optical system, are connected only through the polarized beam splitter 19. That is, the entire optical system has not been considered sufficiently.
(Subject 1)
A first subject of the conventional art is that no compact optical system capable of carrying out various optical information processing such as correlation, convolution, and filtering necessary for the optical information processing apparatus has been achieved.
FIG. 20 shows an example of the most general optical information processing apparatus optical system in which the reflection type SLM 16 is used for filter display.
Light emitted from the coherent light source 10 is changed to collimator light whose wave surface is smoothed by the condensing lens 11, spatial filter 12 and collimator lens group 13 and then projected to the transmission type SLM 14T.
The length between a transmission type SLM 14T in which an input image 141 is displayed and a reflection type SLM 16 in which a filter is displayed is arranged in 2-f (2-focal distance) of a Fourier transformation lens group 15. The length between a reflection type SLM 16 and an image pickup device 18 is arranged in 2-f (2-focal distance) of a Fourier inverse transformation lens group 17.
The transmission type SLM 14T expresses pixel value of the input image 141 by an orientation of liquid crystal molecules in the pixel.
Because information of the input image 141 is modulated to P-polarized light and transmitted without being reflected by the reflection surface 192R of the polarized beam splitter (hereinafter referred to as PBS) 192, the Fourier transformation image 161 is generated on the reflection type SLM 16.
The filter 162 is displayed on the reflection type SLM 16 so that various filterings for optical information processing are carried out.
Because this reflection type SLM 16 modulates information to be read out to S-polarized light, it is reflected by the reflection surface 192R of the PBS 192 and finally a processing result image 181 is obtained on the image pickup device 18 placed near the rear focal plane IFB of the reversed Fourier transformation lens group 17.
FIG. 20 shows an opening as the input image 141, indicating a processing for carrying out bypass filtering to the Fourier transformation image 161 by means of the filter 162, as an example.
FIG. 21 shows a conventional example in which the reflection type SLM 14R is used for displaying the input image 141.
Although this optical information processing apparatus operates in the same way as that shown in FIG. 20, it is different from that of FIG. 20 in that a polarized beam splitter 191 is added to irradiate collimator light.
The collimator light emitted from the collimator lens group 13 is reflected by the reflection surface 191R of the PBS 191 and turned to S-polarized light so as to impinge upon the reflection type SLM 14R.
Next, this collimator light impinges upon the reflection type SLM 14R so as to read out the input image 141.
At that time, information of the input image 141 is expressed in P-polarized light.
Then, this optical information processing apparatus operates in the same way as in FIG. 20 after the reflection type SLM 14R, so that finally the processing result image 181 is obtained on the image pickup device 18.
The same function is secured in a disposition shown in FIG. 22 also.
In this case, the collimator light emitted from the collimator lens group 13 is turned to P-polarized light and beam of light for reading out the input image 141 displayed on the reflection type SLM 14R is turned to S-polarized light.
Then, beam of light having information of the input image 141 is reflected by the respective reflection surfaces 191R, 192R of the PBS 191, 192 and impinges upon the reflection SLM 16.
Filtering is carried out by this reflection SLM 16 so that the processing result image 181 is obtained on the image pickup device 18.
(Subject 2)
A second subject of the conventional art is that the number of the lens elements necessary for the optical information processing apparatus increases.
If the reflection type SLM 16 is used for display of the filter 162, the PBS 192 is necessary for introducing the processing result image 181 to the image pickup device 18.
According to the conventional example shown in FIGS. 20 to 22, this PBS 192 is disposed between the final lens group of the Fourier transformation lens group 15 and the reflection type SLM 16.
At the same time, the PBS 192 is disposed between a first lens group of the reversed Fourier transformation lens group 17 and the front focal plane IFF.
However, if the capacity of a handled image is enlarged, a high optical performance is demanded for the Fourier transformation lens group 15 and reversed Fourier transformation lens group 17.
As a result, there occurs such a problem that the number of the Fourier transformation lens group 15 and reversed Fourier transformation lens group 17 increase so that the total number of the lens elements necessary for the optical information processing apparatus increases enormously.
(Subject 3)
A third subject of the conventional art is that the PBS 192 does not indicate an excellent polarization characteristic.
Here, the PBS has a function for splitting beam of light impinging to S-polarized light and P-polarized light.
For example, in cube type PBS used most generally, the P-polarized light passes through while the S-polarized light is reflected by the reflection surface of the PBS.
If an incident angle of beam of light impinging upon the PBS with respect to the reflection surface of the PBS is substantially 45xc2x0, ratio Ts/Tp between the intensity Tp of the passing P-polarized light component and the intensity Ts of the passing S-polarized light component decreases extremely.
At the same time, a ratio Rp/Rs between the intensity Rs of the reflected S-polarized light component and the reflected P-polarized light component also decreases, so that a very high optical quenching ratio is obtained.
If converging beam of light or dispersed beam of light impinges upon the PBS, although the P-polarized light and S-polarized light are split at a high optical quenching ratio in components near 45xc2x0 in term of incident angle, such a high optical quenching ratio cannot be obtained in beam of light having the other incident angles.
According to the conventional example shown in FIGS. 20, 22, the PBS 192 is disposed between the Fourier transformation lens group 15 on which beam of light is converged and the reflection type SLM 16.
Because information of each pixel of the input image 141 is expressed using polarization of beam of light, if the PBS 192 is disposed at a position like the conventional example, there occurs such a problem that an accurate Fourier transformation image 161 and processing result image 181 cannot be obtained.
Particularly, because a higher NA is demanded for the Fourier transformation lens group as the capacity of handled image increases, in the conventional structure, the problem makes more worse.
(Subject 4)
A fourth subject of the conventional art is that the PBS 191 necessary for indicating the input image 141 on the reflection type SLM 14R does not indicate an excellent polarization characteristic.
If the reflection type SLM 14R is used for displaying the input image 141, the PBS 191 is necessary for irradiation of the collimator light.
According to the conventional example shown in FIGS. 21, 22, this PBS 191 is disposed between the first group of the Fourier transformation lens group 15 and the reflection type SLM 14R.
Although in FIGS. 21, 22, beam of light between the reflection type SLM 14R and PBS 19 is drawn substantially parallel to optical axis, actually, collimator light irradiated on the input image 141 is affected by diffraction at the time of reflection.
This diffraction angle is caused by spatial frequency component of the input image 141 and increases in diffracted light having a higher spatial frequency component.
Therefore, although the PBS 191 functions at a high optical quenching ratio to the diffracted light having a low spatial frequency, there occurs such a problem that an accurate Fourier transformation image 161 cannot be obtained because the function drops if the diffracted light having a high spatial frequency component is applied.
(Subject 5)
A fifth subject of the above described conventional art is that if so-called electric address type SLM is used for the reflection type SLM 16, xe2x88x921 order and +1 order based on the pixel period structure overlap with the processing result image 181, so that an accurate processing result image cannot be obtained.
FIG. 23 shows a case in which the processing result image 181 is not formed accurately due to an influence of the reflection type SLM 16.
To simplify a description, a process of reflection by the reflection type SLM 16 will be described as transmission.
That is, of the diffracted lights generated in the SLM 14 in which the input image 141 is displayed, those caused by the same frequency component become parallel to each other and impinge upon the Fourier transformation lens group 15, so that after transmission, they are focused on a point on the rear focal plane FB of the Fourier transformation lens group 15.
The Fourier transformation lens group 15 has telecentric feature to the FB plane, and the chief ray of lights generated by each spatial frequency impinges perpendicularly on the FB plane.
The processing result image 1810 by the 0 order light not affected by the diffraction of the reflection type SLM 16 is obtained as AoBo on the rear focal plane IFB of the reversed Fourier transformation lens group 17.
Further, the processing result images 181p, 181m obtained by +1 order and xe2x88x921 order diffracted lights by the reflection type SLM 16 are obtained as ApBp, AmBm.
If an influence of diffraction in the reflection type SLM 16 is not considered, as shown in FIG. 23, the processing result images 181p, 181m by the +1 order and xe2x88x921 order diffracted lights overlap with the processing result image 1810 by the 0 order light not affected by the diffraction of the reflection type SLM 16, so that the problem is raised that an accurate processing result image 181 cannot be obtained.
An object of the present invention is to solve the aforementioned first subject and then provide an optical information processing apparatus using a compact optical system capable of carrying out various optical information processing such as correlation, convolution and filtering.
Another object of the present invention is to solve the aforementioned second to fifth subjects and then provide an optical information processing apparatus which uses the reflection type SLM for filter display, the reflection type SLM being capable of carrying out various optical information processings such as correlation, convolution and filtering to a large capacity image at a high precision and reducing the number of necessary components.
To solve the aforementioned first subject, according to a first aspect of the present invention, there is provided an optical information processing apparatus comprising: a coherent light source; a collimate optical system for collimating light from the coherent light source; a reflection type spatial light modulator for returning the collimate light from the collimate optical system toward the collimate optical system; and a Fourier transformation optical system for Fourier-transforming light from the reflection type spatial light modulator and sharing at least part of optical system with the collimate optical system.
To solve the aforementioned first subject, according to a second aspect of the present invention, there is provided an optical information processing apparatus comprising: a coherent light source; a collimate optical system for collimating light from the coherent light source; a reflection type spatial light modulator for returning the collimate light from the collimate optical system toward the collimate optical system; a Fourier transformation optical system for Fourier-transforming light from the reflection type spatial light modulator and sharing at least part of optical system with the collimate optical system; and a polarized beam splitter disposed in an optical interval between a lens including an emission face of the collimate optical system and a lens including an emission face of the Fourier transformation optical system.
To solve the aforementioned second to fifth subjects, according to a third aspect of the present invention, there is provided an optical information processing apparatus comprising: a coherent light source; a collimate optical system for collimating light; a spatial light modulator for displaying an input image; a polarized beam splitter; and a reflection type spatial light modulator for filtering light, the optical information processing apparatus being so constructed that emission light from the coherent light source passes an incident light path comprising the collimator optical system, the spatial light modulator and the polarized beam splitter in this order and reaches the reflection type spatial light modulator, and light reflected by the reflection type spatial light modulator passes a reflection light path which is reverse to the incident light path and reaches the polarized beam splitter, so that the incident light path and the reflection light path are split by the polarized beam splitter, wherein a shared optical system which is at least part of Fourier transformation optical system for incident light and at least part of reversed Fourier transformation optical system for reflection light is disposed in light path between the polarized beam splitter and the reflection type spatial light modulator.
Further, to solve the aforementioned second to fifth subjects, according to a fourth aspect of the present invention, there is provided an optical information processing apparatus comprising: a coherent light source; a polarized beam splitter; and a reflection type spatial light modulator for displaying an input image, the optical information processing apparatus being so constructed that emission light from the coherent light source passes an incident light path comprising the polarized beam splitter and reaches the reflection type spatial light modulator, and light reflected by the reflection type spatial light modulator passes a reflection light path which is reverse to the incident light path and reaches the polarized beam splitter, so that the incident light path and the reflection light path are split by the polarized beam splitter, wherein an afocal optical system for focusing beam of light on the side of the reflection type spatial light modulator is disposed on optical path between the polarized beam splitter and the reflection type spatial light modulator.
Further, to solve the second to fifth subjects, according to a fifth aspect of the present invention, there is provided an optical information processing apparatus comprising: a polarized beam splitter; and a reflection type spatial light modulator, the optical information processing apparatus being so constructed that incident light passes through an incident light path via the polarized beam splitter up to the reflection type spatial light modulator and reaches the reflection type spatial light modulator and a reflection light from the reflection type spatial light modulator passes through a reflection light path which is reverse to the incident light path and reaches the polarized beam splitter, so that the incident light path and reflection light path are split by the polarized beam splitter, wherein 0 order diffracted light component and xc2x11 order diffracted light components of the reflection light do not overlap each other.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.